1. Field of the Invention
The invention relates to a method of thin film processing and, more particularly, it relates to a method of forming an integrated film stack of titanium and titanium nitride with improved reliability.
2. Description of the Background Art
In the manufacture of integrated circuits, a titanium nitride film is often used as a metal barrier layer to inhibit the diffusion of metals into an underlying region beneath the barrier layer. These underlying regions include transistor gates, capacitor dielectric, semiconductor substrates, metal lines, and many other structures that appear in integrated circuits.
For example, when an electrode is being formed for a transistor's gate, a diffusion barrier is often formed between the gate material and a metal that serves as the contact portion of the electrode. The diffusion barrier inhibits the diffusion of the metal into the gate material, which may be composed of polysilicon. Such metal diffusion is undesirable because it would change the characteristics of the transistor, or render it inoperative.
When the underlying region is silicon (Si), it is also desirable to use a titanium/titanium nitride (Ti/TiN) combination as a barrier layer. Since the contact resistivity of Ti to Si is significantly lower than that of TiN to Si, the use of Ti to contact the underlying Si will lead to a lower contact resistance, while the TiN layer will act as an effective barrier against diffusion of metal into the underlying silicon.
Ti and TiN films can be formed by physical or chemical vapor deposition. A Ti/TiN combination barrier layer may be formed in a multiple chamber "cluster tool" by depositing a Ti film in one chamber followed by TiN film deposition in another chamber. When depositing both Ti and TiN using chemical vapor deposition (CVD), titanium tetrachloride (TiCl.sub.4), for example, may be used to form both Ti and TiN films when allowed to react with different reactant gases.
However, when a TiCl.sub.4 -based chemistry is used in such an integrated Ti/TiN film process, a reliability problem is encountered. In particular, it has been found that if the thickness of the Ti film exceeds about 120 .ANG., the integrated Ti/TiN stack structure will either peel off or exhibit a haze that is indicative of poor film quality. This problem can be overcome by special treatment of the Ti film after Ti deposition. For example, if the Ti film is first subjected to a rapid thermal nitrogen (RTN) treatment, the subsequently deposited TiN film will not exhibit film failure in the form of peeling or haze. However, the drawback for this approach is that the additional process step of RTN can only be performed in a separate piece of equipment outside of the cluster tool. If the Ti film is formed from a high temperature CVD Ti process, the need for the additional RTN step decreases the process throughput. In general, an extra process step requiring additional wafer handling and transfer among different equipment translates to extra cost and additional risk for wafer contamination or breakage. Since Ti and TiN deposition are usually performed within different chambers in a cluster tool, the need for an additional step such as RTN would defeat the purpose of using a cluster tool for this Ti/TiN application.
Therefore, a need exists in the art for a method of forming a reliable Ti/TiN diffusion barrier in an integrated process.